1. Field of the Invention
The invention relates to a method of fabricating silicon ingle crystals by the CZ method, particularly to a method of fabricating silicon single crystals with superior gate oxide integrity characteristics.
2. Description of the Related Art
Silicon single crystals are generally produced by the CZ method. In the CZ method, polysilicons are fed into a quartz crucible disposed in a single crystal-fabricating apparatus, and then the raw material is heated and melted by heaters disposed surrounding the quartz crucible. Thereafter, a seed crystal mounted on a seed chuck is dipped into the melt. The seed chuck and the quartz crucible are then rotated in the same or reverse direction while pulling the seed chuck to grow a silicon single crystal of a predetermined diameter and length.
It is well known that as-grown silicon single crystals fabricated by the CZ method comprises octahedral void like defects. These defects are detected with infrared scattering tomography(hereinafter referred as LSTD, LSTD: Laser Scattering Tomography Defect). LSTDs exist in almost all single crystals at a density exceeding 1.times.10.sup.6 pieces/cm.sup.3.
In recent years, with the scaling-down and higher integration of semiconductor devices, the gate oxide integrity has particularly received attention. FIG. 2 shows the relationship between LSTD density and the gate oxide integrity yield, in which when the LSTD density is greater than 1.times.10.sup.6 pieces/cm.sup.3, and the gate oxide integrity yield is below 50%. FIG. 3 shows the relationship between the gate oxide integrity yield and the reliability failure of semiconductor devices (Semiconductor Silicon (1994), 937-986), in which when the gate oxide integrity yield falls below than 50%, and the reliability failure rises significantly. In view of the above, the reduction of LSTD density is the most important issue for the growth of silicon single crystals grown by the CZ method (hereinafter referred as CZ-Si single crystal). It is necessary to reduce the LSTD density of the as-grown CZ-Si single crystal to less than 1.times.10.sup.6 pieces/cm.sup.3.
Up to now, as a method of reducing the density of oxygen precipitates of CZ-Si single crystals, which is associated with the gate oxide integrity, it has been proposed to grow the single crystals at a growth rate of less than 0.8 mm/min (see JP-A-2-267195, JP-A-:unexamined published Japanese application). Also, it has been proposed to grow singe crystals with a fp/G coefficient being greater than 0.25 mm.sup.2 /.degree. C. min., wherein fp (mm/min) is the crystal growth rate and the G (.degree. C./mm) is the temperature gradient along the crystal axial direction in the temperature range from the melting point of silicon to 1300.degree. C.; and the cooling rate in the range from 1150.degree. C. to 1000.degree. C. are less than 2.0.degree. C./min (see JP-A- 8-12493).
According to the fabricating method of silicon single crystals disclosed in JP-A- 8-12493, the fp/G coefficient is used as the parameter for controlling the outer diameter of the ring like oxidation induced stacking fault (hereinafter referred as OSF) that occur in the thermal oxidation process after the processing of wafers distributed in the wafer periphery portion which is not used in the semiconductor device fabricating step, and the cooling rate in the range from 1150.degree. C. to 1000.degree. C. is specified as a parameter for controlling the LSTD density. That is, with respect to the above two objectives, each parameter is specified.
The relation between the defects that occur in the silicon single crystals and the cooling process of the crystal growth has been reported in a large number of reports. And when the crystal pulling rate is V(mm/min), the temperature gradient along the crystal axis within the temperature range from melting point of silicon to 1300.degree. C. is G1(.degree. C./min:G1&gt;0), the temperature gradient along the crystal axis within the temperature range from 1150.degree. C. to 1080.degree. C. is G2 (.degree. C./min:G2&gt;0), the relationship between the parameters of V/G1, V.times.G2 and the crystal defects, has also been reported respectively. First, V/G1 is the parameter for determining the sites where OSF occur in the diameter direction of crystals has been described in Materials Sciences Forum Vols. 196-201 (1995) pp. 1713-1718 and Journal of Crystal Growth 151 (1995) pp. 273-277. It goes without saying that if ring-shaped OSF inside the wafer will have a deleterious effect on the fabrication of semiconductor devices. Even if in the outer region of ring-shaped OSF dislocation may occur and the getting ability of heavy metals may be reduced. Accordingly, commercialized wafers are manufactured with the V/G1 greater than a certain critical value so as to keep the wafers within the inside of the ring-shaped OSF.
Moreover, the determination of the varieties of the so-called generating defects which are the secondary defects, A defects and B defects, caused by the interstitial silicon atom, and the secondary defects, D defects, caused by the vacancies, has also been reported, (Journal of Crystal Growth 59 (1982) 625-643). It is presumed that the above parameters dominate the concentration of the point defects (interstitial silicon atom and vacancy) induced in the growing process, and ore or less influence the formation of the LSTD which is considered as the aggregate of the vacancies.
V.times.G2 is corresponding to the cooling rate of the growing silicon single crystal within the range of 1150.degree. C. to 1080.degree. C. There is a good relation between the cooling rate and the LSTD density, as reported in Materials Science Forum Vols. 196-201 (1995) pp. 1707-1712.
As described above, conventionally the parameters V/G1 V.times.G2 were only controlled respectively so as to satisfy each of objects.